[#4] Standard headers may be included in any order; each may
be included more than once in a given scope, with no effect
different from being included only once, except that the
effect of including <assert.h> depends
on the definition of NDEBUG (see 7.2).

Does "with no effect different" mean:

the includes on lines 1 and 3 have the same effect, so at
line 4 the macro FOPEN_MAX is defined;

the include on line 3 has no effect, so that at line 4
the macro FOPEN_MAX is undefined;

something else ?

Most current implementations wrap the contents of headers
with an "idempotent guard", such as:

This will provide behaviour (2), which I would suggest is the
most desirable.

Furthermore, the concept of scope doesn't apply here, both
because includes happen during preprocessing and because there
is a requirement in the same paragraph that:

If used, a header shall be included outside of any external
declaration or definition,

If the wording is being altered, this would be a good
opportunity to fix this as well.

Suggested Technical Corrigendum

Change the first sentence of 7.1.2#4 to:

[#4] Standard headers may be included in any order; each
may be included any number of times in a preprocessing
translation unit. The second and subsequent occurrences of a
given header shall be ignored, except in the case of
<assert.h> (where the behaviour is
defined in subclause 7.2).

Committee Response

The Committee believe that both answer 1 and 2 are allowed,
and does not see a compelling reason to change this.